While modern medicine has been successful in treating numerous human conditions, a large number still remain outside the scope of effective treatment. Gene therapy is one method of potentially treating diseases that are currently untreatable. However, successful gene therapy requires improvements in delivery of the target gene (1). To increase the safety and efficacy of gene therapy agents, tissue specific promoter elements are being incorporated into the delivery vectors (2). This is particularly true for gene therapy agents developed to treat cancers that originate from a particular tissue or cell lineage (3). While tissue specific promoters restrict expression of the gene therapy agent to target tissues, they often direct only low levels of gene expression within the targeted cells (2). To increase gene expression to therapeutic levels a number of enhancer elements have been used (2). However, only a limited number of tissue specific promoter and enhancer elements have been described thus far, making it important to continue to identify additional tissue specific regulatory sequences that may be developed for gene therapy.
GDEP (gene differentially expressed in prostate cancer, aka., PCAN1), a newly discovered gene with remarkable tissue specificity, is a promising candidate for regulatory analysis because it exhibits a high level of expression that is limited to two tissues, the retina and the prostate (4,5,6). As these two tissues have different origins and disparate functions it is likely that the regulatory mechanisms responsible for expression are not shared in their entirety. In addition, both the retina and prostate are prime targets for gene therapy (7,8).
GDEP expression in prostate tissue has been previously documented. GDEP was initially identified through dbEST data mining for prostate specific genes (4). The high level of prostate specific expression was confirmed using Northern Blot analysis and reverse transcriptase PCR (4,5,6). GDEP is expressed exclusively in the prostate epithelial cells, particularly basal epithelial cells, and not in the surrounding stromal prostate tissue (4,5). GDEP is also expressed in a number of prostate cell lines and this expression is enhanced when cells are grown in the presence of Matrigel suggesting that paracrine factors influence GDEP expression (6). Furthermore, GDEP expression is insensitive to testosterone treatment (5,6) making this gene a potential vehicle for investigating prostate specific but testosterone independent regulation.
In addition to prostate expression, GDEP is highly expressed in neural retinal tissue as well as retinoblastoma cell lines (6). Neural retinal tissue from both male and female donors was tested for GDEP expression to ensure that expression was not sex limited. All tissue exhibited a high level of GDEP transcript when compared to actin using RT-PCR. Retinoblastoma cell lines Y79 and WERI-Rb-1 were also both positive for GDEP expression. In contrast, ARPE-19 (a retinal pigmented epithelial cell line) exhibited no expression of this gene, making this gene specific to the cell types found in the neural retina and not the surrounding retinal tissue (6).
As can be appreciated, GDEP is a promising candidate for regulatory analysis because it exhibits a high level of expression that is limited to two tissues, the retina and the prostate. To date there have been no functional studies of the GDEP promoter. Therefore, gaining an understanding of how GDEP is regulated in the tissue specific context and identifying the sequences responsible for this regulation are important goals. Identification of such sequences would open routes for a wide variety of practical applications including, but not limited to, novel methods for tissue specific gene therapy.